Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/tests/test_turn_flow_integration.py

@@ -1,8 +1,9 @@
-"""End-to-end HTTP tests for undo/retry state revert, driving real turns through
-the actual routes + scripting engine with only the LLM provider mocked.
+"""End-to-end HTTP tests for undo and retry state revert. These tests drive
+real turns through the actual routes and scripting engine, with only the LLM
+provider mocked.
 
-A script's output hook adds 10 gold each turn; we assert the shared scoreboard
-behaves correctly across play / undo / retry.
+A script's output hook adds 10 gold each turn. These tests confirm that the
+adventure's stored gold total stays correct across play, undo, and retry.
 
     python -m pytest tests/test_turn_flow_integration.py -v
 """
@@ -64,7 +65,7 @@ def client(monkeypatch):
     adv_id, user_id = adv.id, user.id
     setup.close()
 
-    # Force a real (non-demo) turn that uses our fake provider.
+    # Force a real, non-demo turn that uses the fake provider.
     monkeypatch.setattr(adventures, "OpenAICompatibleProvider", FakeProvider)
     monkeypatch.setattr(auth, "resolve_provider_config", lambda s: auth.ProviderConfig(
         "http://fake", "k", "test-model", False))
@@ -107,7 +108,7 @@ def test_play_then_undo_reverts_gold(client):
 
     r = client.post(f"/api/adventures/{client.adv_id}/undo")
     assert r.status_code == 200, r.text
-    assert _state(client.adv_id) == {}  # scoreboard rolled back
+    assert _state(client.adv_id) == {}  # gold reverted to zero
 
 
 def test_two_turns_then_undo_reverts_only_last(client):
@@ -123,7 +124,8 @@ def test_retry_does_not_double_apply_gold(client):
     _play(client)
     assert _state(client.adv_id) == {"gold": 10}
 
-    # Before the fix this produced 20 (output hook ran twice); now it stays 10.
+    # Before the fix, this produced 20 because the output hook ran twice.
+    # Now it stays 10.
     r = client.post(f"/api/adventures/{client.adv_id}/retry")
     assert r.status_code == 200, r.text
     assert _state(client.adv_id) == {"gold": 10}